1. Field of the Invention
The present invention relates to a gain control circuit using a metal semiconductor field-effect transistor (MESFET) device of gallium arsenide (GaAs) that bay be used in television tuners. The present invention also relates to a semiconductor device.
2. Description of the Prior Art
Gain control circuits conventionally used in television tuners generally control the gain using a device combining P-I-N diode attenuator and fixed gain amplifier, or by changing the operating point of a dual gate FET device. Cost effective commercial application of P-I-N diodes is impeded, however, by the difficulties of manufacturing a P-I-N structure with stable characteristics from GaAs. When a gain control circuit using dual gate FET devices is used, the drain current (hereinafter referred to as "I.sub.DS ") varies with the change in the operating point. This necessitates, as described in IEEE Transactions on Consumer Electronics (Vol. 35, No. 3, August 1989), a separate circuit to stabilize the DC bias and to stabilize the variation in current consumption of the overall gain control circuit due to the gain control voltage. IEEE is an abbreviation for Institute of Electrical and Electronics Engineers, Inc.
In a semiconductor device integrating devices such as GaAs MESFET devices with high I.sub.DS variation to form a gain control circuit, the DC bias of the drain will fluctuate as the voltage drop changes with the variation in I.sub.DS if a resistance load is used between the power supply and drain of the source ground amplifier. Furthermore, while the drain load resistance must be increased to obtain a sufficient gain, this also produces a larger change in the DC bias of the drain. As a result, a drain-source voltage (hereinafter referred to as "V.sub.DS ") sufficient to assure operation of the FET in the saturation range is set in the conventional semiconductor device when the power supply voltage is low, and it is difficult to simultaneously obtain sufficient gain and a stable DC bias.
In addition, a gain control circuit with a circuit construction which does not require a special power supply to set the gain control voltage and does not require a power supply providing a negative voltage is desirable for general purpose applicability. This type of circuit construction requires that the voltage range needed to obtain both the maximum and minimum gain levels be within the power supply voltage to ground voltage range of the gain control circuit, a requirement which cannot be met with the conventional gain control circuit construction.